Method for testing the strength of the material of cast structures,particularly concrete structures



N v- 1970 P. KlERKEGAARD-HANSEN 3, 8

METHOD FOR TESTING THE STRENGTH OF THE MATERIAL OF CAST STRUCTURES, PARTICULARLY CONCRETE STRUCTURES Filed April 2, .1968

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ATTORNEYS United States Patent Oifice 3,541,845 Patented Nov. 24, 1970 METHOD FOR TESTING THE STRENGTH OF THE MATERIAL F CAST STRUCTURES, PARTICU- LARLY CONCRETE STRUCTURES Peter Kierkegaard-Hansen, Nordahl Griegs Vej 16, Soborg, Denmark Filed Apr. 2, 1968, Ser. No. 718,077 Int. Cl. GOln 3/08 U.S. Cl. 73-95 4 Claims ABSTRACT OF THE DISCLOSURE In a concrete structure, in which the test is to take place, a metal piston, for instance in the shape of a cylindrical block, is embedded in the concrete near one of the structures surfaces. A counter-pressure member is placed to rest on said surface and coaxially to the piston. A force is exerted towards the exterior following the axis of the two pieces, the dimensions of which are such and the spacing between them is such that the force of extraction of the piston causes the concrete to fracture forming a frusto-conical face, the piston and the counter-pressure member forming the small and the large base, respectively, of the truncated cone.

SUMMARY OF INVENTION The invention relates to a method which can be employed for the determination and control of characteristic physical properties of the material in cast structures, in particular concrete structures.

The methods commonly used in performing such tests rely on separate specimens cast simultaneously with the casting of the structure and subsequently subjected to tests carried out in accordance with specific rules. The tests concern primarily the determination or the checking of the compressive strength of the material, which, as a rule, is being determined by axial crushing of cylindrical specimens of given dimensions, however, the tensile strength as well as the shear strength of the material is also tested by means of appropriate methods.

The circumstance that these testing procedures operate with separately produced specimens is disadvantageous and has an inherent uncertainty, since these specimens, during their manufacture, may easily acquire properties that dilfer from those of the actual structure and besides, are also liable to being mistaken for others and to similar errors. Accordingly, suggestions have been put forward, which aim at the requisite tests being carried out on the completed structure itself. Thus, it was proposed to test the tensile strength of the material by means of casting spherical bodies into the structure together with the concrete and to extract them again after the setting of the concrete. The force required for the extraction was then to be taken as a measure of the concretes tensile strength. It has been proved, however, that it is not possible in this manner to obtain a useful correlation between the traction used and the tensile strength of the concrete determined by other methods, which is presumably connected with the circumstance that the area of fracture produced by the extraction of the spherical body varies greatly from test to test in both shape and size, with the result that the stresses become an undefined composition of tensile and shear stresses.

A proposal for the determination of the shear strength has also become known based on extracting a piston cast into the concrete through a coaxial cylinder likewise cast into the concrete and having an inner diameter corresponding to the outer diameter of the piston. However, this method is connected with great practical difiiculties,

since a high degree of accuracy is required when centering the piston with regard to the cylinder and when centering the line of application of force. Moreover, tests show that the values for the shear strength of the concrete measured according to this method lie considerably higher than the values determined in other ways. Presumably, the cause for this lies in the circumstance that the plug to be extracted from the concrete expands when being axially compressed in the hole.

So far as is known, no testing method yielding a reliable measure for the perhaps most important strength property of the materials in question, viz. the compressive strength, other than those involving the testing of separately produced specimen, has so far been available.

It is the purpose of the invention to provide a method based on the resistance of the material against destruction and which can be carried out easily on the very material of the structure, and which hereby renders reliable measures for or check on the most important strength properties of this material, the compressive strength in particular. Furthermore, the method can be carried out simply and inexpensively in practice.

The present method is of the type in which the test is performed by outwards pull on a piston embedded in the material towards the adjacent surface of the structure and the novelty of the invention consists in that the reaction of the force of this pull is transmitted to the structure by means of a counter-pressure member abutting against the surface in question with an area surrounding the projection of the piston on the surface in the direction of the force and which delineates an area of this surface which is several times larger than the projection of the piston. Hereby is achieved that the surface upon which the forces, affecting the part of the material of the structure influenced by the test act, will be precisely defined, so that a well-defined stress distribution is obtained as well as a fracture face, the shape and size of which can be selected at discretion by coordinating the relative dimensions of the piston, the counter-pressure member and the spacing between them. Thus, it is possible to direct the fracture in such a way that the tractional force required for producing the fracture becomes an exponent for a certain property of the material and can be used directly as a measure of same.

Thus, if the dimensions of the piston and of the counterpressure member are selected in such a manner in relation to each other and to the spacing between same in the direction of the pull that together they define a truncated cone with a vertical angle of about 60, a frusto-conical fracture face will ensue with the counter-pressure member and the piston as the large and the small base, respectively, and with a half apex angle of approximately 30, i.e. fracture face, which corresponds wholly to the shape of the fracture face, produced in the axial crushing of a cylindrical specimen for the determination of the compressive strength of the material. Comparative tests have demonstrated that the requisite force of extraction in these circumstances is so close to being proportional to the compressive strength determined by the cylinder compressive strength test that the force of extraction can serve as a measure of the compressive strength of the material.

If, according to another embodiment of the invention, the dimensions of piston and counter-pressure memher and the spacing between these are coordinated in such a way, that piston and counter-pressure member together define a frusto-conical surface with a very large apex angle, i.e. an apex angle of at least such a fiat fracture is formed when the piston is being extracted that the force of extraction constitutes a useful measure of the tensile strength of the material. Conversely, a counter-pressure member, the diameter of which is only slightly larger than that of the piston, can entail forces of extraction that constitute useful measures for the strength of the material determined by means of triaxial tests.

In the following the invention is described in greater detail while referring to the drawing, which shows partly diagrammatically a sectional view of a concrete structure which is being tested by the method according to the invention.

In the drawing, 1 designates a part of the concrete structure, the surface of which is designated with 2. A piston 3, which is shaped like a cylindrical metal block and which in the centre is provided with a tapped hole, is cast into the concrete, the piston having been secured to the form by means of a filling body, which, after the forms removal has been screwed out of the piston and which left a hole 4. A pull rod has been inserted through this hole and screwed down in the piston 3 and is during the test subjected to a tractional force T, which is directed in an outward direction.

Around the rod 5 and approximately coaxially with same a counter-pressure member 6 in the form of a ring is arranged, the end face of which tests on the surface of the concrete 2 and the inner diameter of which is adapted to the diameter of the piston 3 as Well as to the depth, at which same is embedded in the concrete, so that the fracture face, which is determined by the circumference of the piston 3 and the inscribed circle of the counter-pressure member 6, assumes the shape of a truncated cone, half of the apex angle of which measures about 30. On account of this, the fracture faces attain substantially the same shape as one half of the well-known hour-glass-shaped fracture faces, which are produced in compressive strength tests of cylindrical specimens, and tests have shown that the tractional force T required for causing the fracture in these circumstances, is in the main proportional to the compressive strength determined in such a compressive strength test of cylindrical specimen of the same concrete. It is in this connection of importance that the reaction R of the force of traction is defined exactly with regard to the piston 3 by means of the counter-pressure member 6, whereby the specific desired stress distribution, which, at the instant of fracture leads to the shown frusto-conical shaped fracture face is positively enforced.

As already indicated, the shape and size of the fracture face can be varied considerably by varying the absolute and relative values for the size of the piston 3, the counter-pressure member 6 and the spacing between these; it is only important that these dimensions be kept within such limits that a stress distribution is developed, which, at the instant of fragture leads to a fracture face which is determined by the circumference of the piston and the inner circumference of the counterpressure member.

By variation of these dimensions, as explained above, measures of other properties of the material than the pure compressive strength can be obtained.

The piston, the counter-pressure member and the direction of the pull should of course be coaxial as far as possible, it has, however, turned out that the requirements for accuracy in this respect are no greater than can be easily fulfilled in practice on the building site.

The simplest shape of the piston and of the counterpressure member is the circular one, dispensing with any need for relatively orientating the piston and the counter-pressure member, yet nothing prevents the piston and the counter-pressure member from having another shape, e.g. square, rectangular or any other polygonal shape. i l

The method can be carried out, as indicated in the drawing, also in its application in practice up to the complete fracture, since the craters produced are of no importance to the strength of the entire structure and since they can easily be filled in. In practice, it is thus possible to work with a piston diameter and an embedding depth of only approximately 25 mm. and a diameter of the counter-pressure member of approximately 55 mm.

If it is only desired to check whether the material satisfies a specified requirement as to its strength, it is sufficient to lead to a pull up to the corresponding force of traction. When using statistical quality control, it is also possible to operate with a pull smaller than the one corresponding to the specified strength and thus to avoid a fracture of the material in most cases. Incidentally, it is thus possible to repeat the check at later dates, e.g. for the purpose of testing the durability of the material.

What I claim is:

1. Apparatus for testing the strength of structures cast from material such as concrete, the apparatus comprising in combination, a piston adapted to be embedded in cast material whose strength is to be tester, a pull member having one end adapted to be connected to said piston for purposes of applying a pulling force on said piston along the axial direction of said piston and pull member, and an annular counter pressure member adapted to be applied against a surface of a structure to be tested around said pull member to act an an abutment opposing the force on the structure exerted *by the piston as a result of the pulling force through said pull member, said annular counter pressure member enclosing an area several times larger than the area on the face of said piston, and being axially spaced from said piston when in use such that they define a truncated cone having an apex angle of at least about 60.

2. The apparatus defined in claim 1 further including means on said one end of said pull member and on said piston for releasably securing said pull member to said piston.

3. Apparatus defined in claim 2 wherein said last defined means includes screw threads.

4. In the combination including a cast structure of concrete or the like having a piston embedded therein and a pull member fixed centrally to said piston and projecting generally normally therefrom beyond a surface of said cast structure; the improvement comprising an annular counter pressure member secured against said surface of the structure concentrically about said .pull member to act as an abutment opposing forces on the structure resulting from pulling the piston outwardly through said pull member, said annular counter pressure member surrounding and defining an area on said surface of said structure several times larger than the projected area of said piston, and being axially spaced from said piston when in use such that they define a truncated cone having an apex angle of at least about 60.

References Cited UNITED STATES PATENTS 3,182,493 5/1965 Patterson et al. 73-95 FOREIGN PATENTS 1,215,958 5/1966 Germany.

JERRY W. MYRACLE, Primary Examiner U.S. Cl. X.R, 73101 

